In recent years, a mobile communication system using a digital modulation method has vigorously been researched and developed. If a high efficiency amplifier is employed in the transmission system in order to attempt to save power at a radio transceiver, nonlinear distortions frequently result therefrom. As a means, there is a method for compensating nonlinear distortions of amplitude and phase with reference to a distortion compensating table, by using an amplitude value of transmission base band signals.
A description is given of a conventional transmission apparatus which compensates nonlinear distortions by using such a method.
FIG. 10 is a main block diagram of a conventional transmission apparatus. In FIG. 10, amplitude calculating section 1001 calculates amplitude information 1010 of transmission signals 1009 (transmission digital quadrature base band of I and Q channels), and compensation table 1002 outputs distortion compensating coefficient 1011. The distortion compensating section 1003 outputs a distortion compensating signal 1012 according to the transmission signal 1009 and distortion compensating coefficient 1011. The outputted distortion compensation signal 1012 is quadrature-modulated by quadrature modulating section 1004, and the modulated signal 1013 is amplified by amplifier 1005, wherein amplified RF signal 1014 is outputted.
Furthermore, demodulating section 1007 demodulates feedback RF signal 1016 fed back from coupler 1006 to feedback base band signal 1017, and estimating section 1008 updates the distortion compensation coefficient of compensation table 1002 on the basis of a distortion compensation coefficient 1018, transmission signal 1009 and feedback base band signal 1017. Through the abovementioned actions, amplified RF signal 1015 for which nonlinear distortion compensation is carried out is outputted from the coupler 1006.
In addition thereto, as a method to compensate nonlinear distortions in the transmission system, there is a method having a digital filter to which a ROM (Read-only-memory) is attached, wherein compensation is carried out by applying in advance distortions to compensate nonlinear distortions generated in an amplifier by the digital filter.
The main block diagram of a conventional transmission apparatus in which this method is employed is shown in FIG. 11. Hereinafter, a description is given to this apparatus.
In FIG. 11, digital filter 1102 in which digital signals 1101 is inputted gives to the digital signal 1101 distortions to compensate nonlinear distortion components generated in radio frequency power amplifier 1105, by using distortion information stored in a ROM in advance.
The digital signal 1101 to which distortion to be compensated is given is digital-analog converted and modulated by quadrature modulating section 1103 and inputted into the radio frequency power amplifier 1105 via transmission section 1104. In the radio frequency power amplifier 1105, since distortion of the inputted digital signal are compensated in advance, distortions generated at the radio frequency power amplifier 1105 are cancelled by those for compensation.
Furthermore, instead of ROM, there is still another method to compensate nonlinear distortions, in which a RAM (Random Access Memory) having compensation coefficients stored therein in order to compensate nonlinear distortion components is used, by varying the compensation coefficients of the RAM in compliance with amplitudes of digital signals.
Furthermore, Unexamined Japanese Patent Publication No. 290321 of 1992 discloses a method for controlling actions of a digital filter by feeding outputs of a radio frequency power amplifier back to the digital filter.
However, in the conventional example shown in FIG. 10, it is necessary that signals having the maximum amplitude is suppressed less than the maximum output of amplifier 1005, and this results in a lowering of efficiency in the amplifier 1005.
In the conventional example shown in FIG. 11, since it is necessary to provide a memory table such as a ROM or RAM in which compensation coefficients are stored to compensate nonlinear distortion components, and this results in an increase of the scale of transmission circuits themselves.